1. Field of the Invention
The present invention relates to a method used in a communication device in a wireless communication system, and more particularly, to a method of handling data transmission and reception in dual connectivity.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) in Release 12 studies small cell enhancement. Small cells using low power nodes are considered promising to cope with mobile traffic explosion, especially for hotspot deployments in indoor and outdoor scenarios. A low-power node generally means a node whose Tx power is lower than macro node and BS classes, for example Pico and Femto eNB are both applicable. Small cell enhancements for E-UTRA and E-UTRAN will focus on additional functionalities for enhanced performance in hotspot areas for indoor and outdoor using low power nodes.
In addition, 3GPP in Release 12 proposes dual connectivity for increasing user's throughput. Dual connectivity to macro and small cells may be served by different eNBs, linked with non-ideal backhaul, e.g., there may be an eNB in charge of a cluster of small cells in a hotspot area. Therefore, UE may be served by multiple eNBs when it is in dual connectivity mode.
Please refer to FIG. 1, which illustrates protocol structure of a macro eNB, small eNB, and a UE in dual connectivity. In dual connectivity, the UE connects to the macro eNB and small eNB. The downlink data of the radio bear RB1 starts to be distributed by the macro eNB, and is transmitted to the UE by the macro eNB and small eNB. In detail, there is a centralized packet data convergence protocol (PDCP) entity at the macro eNB for both macro and small eNB, and thus security and header compression is controlled by the macro eNB. The PDCP entity of the macro eNB receives downlink data from the radio bear RB1 established by the radio resource control (RRC) entity, and transmits the downlink data to the radio link control (RLC) entity of the macro eNB toward the UE via a component carrier. Meanwhile, the PDCP entity of the macro eNB distributes the downlink data to the RLC entity of the small eNB toward the UE via another component carrier. The component carrier and the other component carrier may belong to different frequency bands. In other words, downlink data carried by the radio bear RB1 is distributed between PDCP and RLC entities, so that traffic QoS is balanced between macro eNB and small eNB as well. On the other hand, the UE includes one PDCP entity corresponding to two RLC entities, wherein a first RLC entity receives the downlink data from the macro eNB and a second RLC entity receives the downlink data from small eNB. Then, the two RLC entities deliver the downlink data to the one PDCP entity corresponding to the radio bear RB1. In reverse, the UE may perform uplink data transmission to the macro eNB via a component carrier and small eNB via another component carrier with the abovementioned protocol structure. Note that, according to the characteristic of a radio bear mapping to the RLC entities, the RLC entities may be in an acknowledged (AM) mode or Unacknowledged (UM) mode for data transmission. The functionality of PDCP/RLC/MAC/PHY entities should be well-known in the art, so it is omitted herein.
In addition, based on 3GPP TS 36.331 V 11.3.0 specification, the UE shall initiate a RRC connection re-establishment procedure when one of the following conditions is met: upon detecting radio link failure, handover failure, mobility from E-UTRA failure, integrity check failure indication from lower layers or an RRC connection reconfiguration failure. Upon initiation of the RRC connection re-establishment procedure, the UE shall stop timer T310 if running, suspend all RBs except SRB0, or reset MAC. Moreover, the UE shall re-establish PDCP for SRB1, or re-establish RLC for SRB1 when the UE receives an RRCConnectionReestablishment message. On the other hand, the UE considers radio link failure upon T310 expiry, upon random access problem indication from MAC while neither T300, T301, T304 nor T311 is running or upon indication from RLC that the maximum number of transmissions has been reached.
Furthermore, based on 3GPP TS 36.323 V11.2.0 specification, when upper layers request a PDCP re-establishment, for radio bearers that are mapped on RLC AM, the UE shall compile the PDCP status report as indicated below after processing the PDCP Data PDUs that are received from lower layers due to the re-establishment of the lower layers, and submit it to lower layers as the first PDCP PDU for the transmission if the radio bearer is configured by upper layers to send a PDCP status report in the uplink, by setting the FMS field to the PDCP SN of the first missing PDCP SDU, if there is at least one out-of-sequence PDCP SDU stored, allocating a Bitmap field of length in bits equal to the number of PDCP SNs from and not including the first missing PDCP SDU up to and including the last out-of-sequence PDCP SDUs, setting as ‘0’ in the corresponding position in the bitmap field for all PDCP SDUs that have not been received as indicated by lower layers, and optionally PDCP SDUs for which decompression have failed, indicating in the bitmap field as ‘1’ for all other PDCP SDUs. In a word, the PDCP Status Report is used for requesting retransmission of PDCP SDUs, and is transmitted from the receiver to the transmitter in order to inform the transmitter about the PDCP PDUs that were received or not received by the receiver PDCP entity, such that non-received PDCP SDUs can be retransmitted and received PDCP SDUs need not be retransmitted.
However, the applicant notices a problem associated to RRC connection re-establishment procedure in dual connectivity. As abovementioned, in dual connectivity, a PDCP entity of the UE corresponds to a first RLC entity for data reception/transmission from/to the small eNB and a second RLC entity for data reception/transmission from/to the macro eNB. When the first RLC entity reaches RLC maximum number of transmissions or a radio link failure occurs on a first connection to the small eNB, the UE performs a RRC connection re-establishment procedure even when a second connection to the macro eNB, where a plurality of RLC PDUs of the second RLC entity are transmitted or received, does not have radio link failure. In this situation, the RLC PDUs transmission of the second RLC entity to the macro eNB is stopped due to the RRC connection re-establishment procedure for connection recovery. Similarly, the RLC PDUs reception of the second RLC entity from the macro eNB is also stopped due to the RRC connection re-establishment procedure for connection recovery.